hooking up 2008 Lund Rebel starting battery

[stubblejumper]

I unhooked both batteries to charge them,and am now I am not sure on how to hook up the starting battery properly.The large cables are no problem,as are the two smaller black wires and red wires that are paired together on a single lug,however there is a third small black wire that I don't remember unhooking.Do I hook that wire up to the negative or positive terminal?